A variety of techniques are used to stack packaged integrated circuits into a module. Some require that the circuits be encapsulated in special packages, while others use circuits in conventional packages. Both leaded and BGA type packaged integrated circuits (ICs) have been stacked. Although BGA packaging has become widely adopted, leaded packages are still employed in large volumes in low cost applications such as, for example, flash memory, which typically is packaged in thin small outline packages otherwise known as TSOPs.
When leaded packages such as TSOPs are stacked, a variety of techniques have been employed. In some cases, the leads alone of packaged circuits have been used to create the stack and interconnect its constituent elements. In other techniques, structural elements such as printed circuit boards (PCBs) are used to create the stack and interconnect the constituent elements.
Circuit boards and rail-like structures in vertical orientations have been used for years to provide interconnection between stack elements. For example, in U.S. Pat. No. 5,514,907 to Moshayedi, a technique is described for creating a multi-chip module from surface-mount packaged memory chips. The devices are interconnected on their lead-emergent edges through printed circuit boards oriented vertically to a carrier or motherboard that is contacted by connective sites along the bottom of the edge-placed PCBs. The PCBs have internal connective rail-like structures or vias that interconnect selected leads of the upper and lower packaged memory chips. Japanese Patent Laid-open Publication No. Hei 6-77644 discloses vertical PCBs used as side boards to interconnect packaged circuit members of the stack. In U.S. Pat. No. 5,266,834 to Nishi et al., one depicted embodiment illustrates a stack created by selective orientation of the leads of particularly configured stack elements, while in U.S. Pat. No. 5,343,075 to Nishino, a stack of semiconductor devices is created with contact plates having connective lines on inner surfaces to connect the elements of the stack. Another technique for stacking leaded packaged ICs with carrier structures or interposers oriented along lead bearing sides of packaged devices such as TSOPs is disclosed by the present assignee, Staktek Group L.P., in U.S. Pat. No. 6,608,763 issued Aug. 19, 2003, to Burns et al., which is incorporated herein by reference for all purposes.
Many of the previously cited and known techniques for using PCBs and similar interposer structures for stacking leaded packaged devices into modules have evolved to meet the increased connective complexity presented by, for example, stacking memory components that have two or more chip enables per packaged device. Connectivity complexities, however, can arise in any applications where there is a need to connect non-adjacent leads of the module ICs. In some cases, this evolution has included use of interposer designs that employ four metal-layer designs to implement the more complex connection strategies required by more complex devices. Size limitations and other factors applicable to packaged IC stacking, however, have led to complexities in via and connection strategies. For example, trace routing and other connective requirements for interposers or carrier structures used in many applications may require the use of buried vias and/or blind vias. In various applications, the micro vias are used for blind vias. The use of multi-layer PCBs with buried vias and blind vias to address complex routing and other connective demands, however, increases costs and may present quality issues due to tight tolerances required.
What is needed, therefore, is a system and method for stacking leaded packaged devices with multi-layer interposer or carrier structure technologies that are easily understood and implemented with simpler and more reliable techniques and materials, but still implement more complex connection strategies.